Inborn errors of metabolism offer an opportunity to investigate the effects of an enzymatic lesion on cellular metabolism and biosynthetic capacities. Our goal is to understand at the cellular level the mechanism by which heritable diseases give rise to abnormal phenotypes. Inborn errors of metabolism will be investigated in cell culture since this system provides a means of studying cellular metabolism in a controlled external environment unaffected by interactions with other cell types. Moreover, molecular mechanisms are more easily investigated in this model. For example, cell hybridization using Sendai virus has been used in our laboratory to study complementation that is restoration of enzyme activity, when two different mutants having the same enzymatic deficiency are fused together forming a heterokaryon. Cell culture provides an opportunity to study enzyme regulation in normal and mutant cells by altering the composition of the medium to either accentuate or mitigate the enzymatic activity. For example, the cyst(e)ine content of medium affects the activity of certain membrane bound enzymes including the branched chain keto acid (BCKA) decarboxylase. These studies might not only contribute to a better understanding of enzyme control, but also might prove useful in detecting carriers of the mutant gene and in developing rational methods of therapy. Comparison between the biochemical and physiologic abnormalities found in culture and those observed in vivo may contribute to an understanding of the variable expression and genetic heterogeneity observed with many of these disorders. Moreover, subtle differences in the regulation of metabolic pathways, percursor levels, and enzyme activities may be among the important aspects of genetic diversity which contribute to susceptibility to a wide variety of disease. The facilities provided by this grant will continue to support prenatal diagnosis and a repository of tissue culture cells derived from patients with inborn errors of metabolism.